1. Field of the Invention
The present invention relates to an x digital subscriber line (hereinafter referred to as an xDSL) modem which comprises a line characteristic estimating function for estimating characteristics of a telephone line connected thereto, when broadband communication services using an xDSL technology are provided.
The xDSL technology is capable of transmitting data of several mega bits per one second at a high speed by use of a metal wire such as an existing telephone wire, and hence the xDSL technology has lately attracted a considerable attention as a communication system in stead of an analog modem. The xDSL technology includes HDSL (high bit rate DSL) technology, SDSL (single line DSL) technology, RADSL (rate adaptive DSL) technology, VSDL (very high bit rate DSL) technology, and ADSL (asymmetric DSL) technology; and among these technologies, the ADSL technology is particularly attractive because its asymmetric transmission capacity is suitable for an Internet access and standardization of technical specifications is advanced. Popularization of a broadband communication service using the ADSL modem has been expected.
2. Description of the Related Art
FIG. 10 shows a constitution example of a communication system which provides broadband communication services by use of an ADSL technology.
Referring to FIG. 10, an ADSL modem 411 and a telephone set 412 installed in a subscriber's home are connected to a subscriber line through a splitter 413, respectively. The subscriber line is connected to a splitter 401 installed in a central office, and signals that have arrived at the central office through the subscriber line are divided by the splitter 401 into a signal of a frequency band used in a plain old telephone system (POTS) and a signal of a frequency band used in the xDSL technology. Analog signals for POTS are delivered to a local switch 402.
Furthermore, in the central office shown in FIG. 10, an ADSL modem 403 is installed by a provider providing broadband communication services by the ADSL (hereinafter simply referred to as broadband communication services). The ADSL modem 403 faces the ADSL modem on the subscriber side. The modulated-analog signal of the frequency band separated by the above-described splitter 401 is converted to a digital data signal by the ADSL modem 403, and thereafter is delivered to a router 404 (or an ATM switching system). Then, the digital signal data is delivered from the router 404 to a server of the provider through a predetermined data channel. Moreover, the digital data signal sent to a user from the server of the provider is delivered to the ADSL modem 403 from the router 404 through a reversed path in the data channel, and is converted to a modulated-analog signal by the ADSL modem 403. The modulated-analog signal is then sent out to the subscriber line through the splitter 401.
In the conventional broadband communication service, when a contract is made between a subscriber of a telephone system provided by NTT (Nippon Telegraph and Telephone company) and a provider providing a broadband communication service, a request for a construction is first issued to NTT from the provider. Subsequently, a work in which a connection of a subscriber line of a customer with the local switch 402 provided in the plain old telephone system (POTS) is changed to that with the ADSL modem 403 is performed in a central office by NTT. Thereafter, the provider sets up an ADSL modem exclusively used for the broadband communication services provided by the provider in a subscriber's home, and then performs a connection check of the ADSL modem.
As a result of this connection check, when it is proved that a good service quality is acquired in the subscriber line connected to the subscriber's home, the subscriber can accept the provision of the broadband communication service by use of the ADSL modem.
On the contrary, when it is proved that a predetermined service quality cannot be acquired in the subscriber line connected to the subscriber's home, on the subscriber line a worker sent from the provider performs an operation for measuring characteristics of the subscriber line in order to investigate causes leading to deterioration of the service quality.
Generally, as the principal cause which deteriorates the service quality in the ADSL modem, assigned are transmission loss in proportion to a line length of the subscriber line between the subscriber's home and the central office, and cyclic noises accompanied by provision of time compression multiplexing-ISDN services in Japan. Accordingly, by measuring the transmission loss and the existence of the cyclic noises on the subscriber line and an intensity of the cyclic noises, it is possible to assign the cause of the deterioration of the service quality. Depending on the assigned cause and a degree thereof, it can be decided whether or not the provision of the broadband communication services is possible, and a necessary countermeasure can be decided if the provision of the broadband communication services is possible.
When, based on the measurement results of the above-described line characteristics, it is decided that the provision of the broadband communication services is possible, a necessary countermeasure is taken, which is performed, for example, in such a manner that the provider makes a request to NTT for changing a connection of the subscriber line connected to the subscriber's home. After the service quality required for the services is secured, the provision of the broadband communication services is begun.
Contrary to this, the service quality cannot sometimes be expected to be improved even by changing the connection of the subscriber line. For example, in the case where the subscriber line to which the service is to be provided and a subscriber line which is producing a crosstalk causing the frequency noises are located very closely to each other, particularly the case where these subscriber lines constitute the same quad comes under the above-described situation.
In such a case, the provider consults with the customer, and decides whether to provide services with limited transmission capacity or to abandon the provision of the broadband communication service itself.
As a technology for achieving coexistence of ADSL and ISDN, for example, there have been one used by applying Annex. C to G. lite and G. dmt that are international standard specs by ITU-T. Concerning G. lite and G. dmt, Annex. C respectively prescribes a spec that adjusts a transmission rate by ADSL so as to synchronize with clocks of ISDN having 400 Hz. In Annex. C, two kinds of modes are prescribed. One is a DBM mode that makes the transmission rate low during the period when a near-end crosstalk power is large; and the other is a FBM mode that performs a transmission by using only the period when the near-end crosstalk power is small. In any case, since the transmission capacity during the period when the near-end crosstalk power is large is drastically restricted, a decrease in the transmission capacity as a whole is inevitable. However, interference by ISDN can be suppressed to the minimum.
As described above, in the conventional procedures in which the broadband communication service is set up, it is unclear whether or not the service quality in the subscriber line of the customer satisfies the required conditions for providing the broadband communication services, until the ADSL modem is installed in the customer's home and the connection check of the ADSL modem is performed. Moreover, even if the causes deteriorating the service quality are assigned by performing the work for measuring the line characteristics for the subscriber line, it is not always possible to remove the causes. Specifically, there is a possibility that the provider does not come to provide the service in spite of costs and man-hours spent until then.
Recently, a case is observed, in which the provider send the ADSL modem to the customer, and entrusts the work for setting up the ADSL modem and the work of the connection test for the ADSL modem, thus attempting to decrease man-hours concerning the set-up work. However, also in this case, when the predetermined level of service quality cannot be acquired by the subscriber line of the customer, as a matter of course, the provider must send the worker to allow the worker to measure the line characteristics. Accordingly, the provider pays considerable costs and performs the considerable number of the working steps.
As described above, in the conventional broadband communication services, since no countermeasure to previously obtain various characteristics of the subscriber line used for services to be contracted, a risk that the provider must bear for setting up the broadband communication service is very serious compared to an income earned by a service contract. This has been a large hindrance from the viewpoint of propagation of the broadband communication service using the xDSL technology.